1. Field of the Invention
The present invention relates to a composite diaphragm for a speaker having a boron layer formed on a surface of a titanium foil, and more particularly to a composite diaphragm for a speaker which significantly increases bonding strength between the titanium foil and the boron layer.
2. Description of the Prior Art
The requirements for a good diaphragm material generally include light weight, high rigidity, high elasticity, high workability and appropriate internal loss and damping factor. Since those requirements compete with each other, it is almost impossible to meet all of the requirements with a known material. In one proposed approach, the diaphragm is made by forming titanium (Ti) foil, which is relatively light and has relatively high elasticity and good workability, into a shape of diaphragm and forming on the surface thereof a boron (B) layer, which is light and has very high elasticity by P.V.D. (physical vapor deposition) technique or C.V.D. (chemical vapor deposition) technique so that the resulting diaphragm has both mechanical property of titanium and high elasticity of boron. However, since the solid solubility between titanium and boron is so low that diffusion layer is hardly formed at the interface, the resulting diaphragm cannot be practically used because of poor bonding between the titanium foil and the boron layer. To resolve the above problem, the following treatments (1) to (3) have been adopted to enhance the bonding strength, but with each it remains difficult to attain satisfactory bonding.
(1) Treatment for cleaning the surface of the titanium foil. PA1 (2) Heat treatment of the titanium foil during the formation of the boron layer. PA1 (3) Heat treatment for forming a diffusion layer of the titanium foil and the boron layer.